1. Field of the Invention
The present invention relates to an image-erecting viewing optical system having a large apparent visual angle and a low magnification.
2. Description of the Prior Art
An image-erecting viewing optical system used in, for example, a binocular or a monocular, generally includes an objective optical system, an image-erecting optical system, and an eyepiece optical system, in this order from the object. In an image-erecting optical system having a wide real field-of-view at a low magnification, there is a need to make the focal length of the objective optical system shorter; however, in an optical arrangement in which the objective optical system is provided in front of the image-erecting optical system, the focal length of the objective optical system has to be made longer by the optical path length of the image-erecting optical system. Consequently, the focal length of the eyepiece optical system has also to be made longer, so that miniaturization of the entire optical system becomes difficult.